The present invention generally relates to a conductive filler and a method of making the filler. The filler is adapted for use in electrical coils, such as Roebel windings as a Roebel filler to fill interstices between the windings and a casing wall. The filler of the present invention is also useful in other applications where a flexible and/or compressible conductive material is needed. For example, the present filler can be used in the construction of an electrically shielded cabinet.
Various techniques for filling voids between parts in electrically conductive devices are known. Unfortunately, many prior art techniques do not succeed in completely filling the voids and/or suppressing electrical discharge across the voids. Many void fillers act as a dielectric and allow a voltage to be impressed across the filler. Failure to fill the voids or at least suppress discharge will result in undesirable arcing between the components. Arcing leads to diminished efficiency and diminished life expectancy of the device.
An example of a conductive device where voids are present is a high voltage coil havipg windings that are intertwined in a braid-like fashion to form a Roebel bar. Roebel bars, or Roebelled windings, have a highly discontinuous surface. This surface has a great number of voids, or interstices, which must be properly filled in order to reduce mechanical and electrical stresses. U.S. Pat. No. 5,175,396 dated Dec. 29, 1992 to Emery, incorporated herein by reference, discloses such a Roebel bar. The U.S. Pat. No. 5,175,396 discloses a prior art void filler made from Dacron felt impregnated with epoxy. The U.S. Pat. No. 5,175,396 is directed to providing a void filler made from an insulating layer of mica paper and B stage epoxy. A semiconductive layer, preferably a paste of carbon filled epoxy, is placed between the inner insulating layer and a groundwall.
Other filler materials have been used to fill voids in electrical coils. Discussion of a resin rich felt material may be found in U.S. Pat. No. 5,633,477 dated May 27, 1997 to Smith. Discussion of an inert filler material and a pyrolyzed glass fiber layer electrically coupled to the strands of a coil may be found in U.S. Pat. No. 5,066,881 to Elton. These fillers, and other prior art fillers and pre-pegs, are often difficult to install in high voltage coils and/or are not suitable foruse in other applications, such as in the construction of an electrically shielded cabinet.
The present invention overcomes these disadvantages by providing an electrically conductive filler, the filler having a substantially non-conductive core and a conductive layer. The conductive layer is wrapped around the core to form a closed loop thereby establishing conductivity from a bottom of the filler to a top of the filler by paths on each of two sides of the filler.
According to another aspect of the invention, the conductive layer is wrapped around the core such that the top of the filler is provided with two laminations of the conductive layer.
The present invention provides a method of making a web of conductive filler by placing a web of core material onto an interior surface of a web of conductive layer material. The webs of core material and conductive layer material are directed through a forming station. The forming station turns first and second edges of the conductive layer material upward, folds the first edge of the conductive layer material over the core material, and folds the second edge of the conductive layer material over the first edge of conductive layer material.